The present invention relates to micro-electro mechanical system (MEMS) devices, and more particularly, to MEMS mirrors adapted to operate safely under high applied voltages.
A MEMS device is typically formed, in part, using well known integrated circuit (IC) fabrication processes. One such device is a gimbaled mirror adapted to pivot in response to electrostatic actuation. The mirror's motion is used to redirect light beams. Electrostatic actuation is often achieved via capacitive action between electrodes formed on a planar surface of a substrate and the mirror positioned above the electrode. To obtain a relatively large range of angular motion, the mirror is suspended at a relatively large distance above the electrodes. Accordingly, a relatively large voltage is often necessary to cause the actuation.
In practice, the maximum voltage that can be applied is limited by the well understood voltage breakdown effects. For example, if two flat electrodes are separated by a uniform gap, the voltage at which the ambient gas breaks down is described by the well known Paschen's curve, which is shown in FIG. 1. As seen from FIG. 1, the exact curve depends on the type of ambient gas, but the general shapes are similar. Breakdown is characterized by an uncontrolled discharge of electrical energy, which can cause severe physical damage to any surfaces exposed to the discharge.
In addition to breakdown of the ambient gas, breakdown can also occur along surfaces. If two conductors are separated by an insulator, breakdown can occur along the surface of the insulator. Unlike Paschen breakdown, surface breakdown is not a simple function of separation, but is a more complex function of the linear path length of the insulator and is strongly influenced by surface contaminants.
Whether breakdown occurs through the gas or on the surface, safe operation of MEMS devices using electrostatic actuation must occur below voltages that would result in breakdown. This constraint places practical limits on the design of electrostatic MEMS devices such as the MEMS mirror mentioned above.
Another common problem encountered in electrostatic MEMS devices is control of the surface potentials between the electrodes and various insulating surface layers. Surface potentials on dielectric surfaces are prone to drift over time due to charge migration along dielectric surfaces between the electrodes. This can cause serious problems regarding repeatability of the mirror positioning.
The conduction characteristics of these surfaces are inherently unstable due to sensitivity to temperature, moisture and other environmental factors. They can also be affected by electromagnetic radiation (light), which can be time dependent, depending on the application, contributing to system crosstalk. The conductivity of these surfaces is also strongly affected by impurities and process steps and materials used in the deposition and etching of the surfaces. All of these factors combined contribute to a loss of control of the surface potentials that contribute to the forces and torques applied to the actuable elements resulting in an unreliable and uncontrollable device.